R-1. T. H. James, "The Theory of the Photographic Process," 4th edition, Macmillan, New York, N.Y. 1977.
R-2. N. Itoh, J. Soc. Photogr. Sci. Tech., Japan, 52, 329 (1989); Y. Toda, in "Photographic Gelatin," H. Amman-Brass and J. Pouradier Ed., International Working Group for Photographic Gelatin, Fribourg, 1985.
R-3. P. Bagchi, J. Colloid and Interface Sci., 47, 86 (1974).
R-4. M. D. Sterman and J. L. Bello, "Chain Extended Gelatin," U.S. patent application Ser. No. 612,370 filed Nov. 14, 1990.
R-5. P. Bagchi, ACS Symp. Ser., 9, 145 (1975).
R-6. P. Bagchi, J. T. Beck, and L. A. Crede, "Methods of Formation of Stable Dispersions of Photographic Materials," U.S. Pat. No. 4,990,431 (1991).
R-7. J. I. Cohen, W. L. Gardner, and A. H. Herz, Advan. Chem. Ser., 45, 198 (1975).
R-8. P. Bagchi and S. M. Birnbaum, J. Colloid and Interface Sci., 45, 198 (1975).
R-9. H. A. Hoyen and R. M. Cole, J. Colloid Interface Sci., 41, 93 (1972).
R-10. R. R. Irani and C. F. Callis, "Particle Size: Measurement, Interpretation and Application," John Wiley, London, 1963.
R-11. B. Chu, "Laser Light Scattering," Academic Press, New York, 1974.
R-12. Anonymous, "Photographic Silver Halide Emulsions, Preparations, Addenda, Processing and Systems," Research Disclosure, 308, pp. 993-1015 (1989).
R-13. S. Nagamoto and K. Hori, "Silver Halide Light-Sensitive Material," U.S. Pat. No. 4,266,010 (1981).
R-14. S. Nagamoto and K. Hori, "Silver Halide Photographic Materials With Surface Layers Comprising Both Alkali and Acid Processed Gelatin," U.S. Pat. No. 4,021,244 (1977).
R-15. K. Hori and S. Nagamoto, "Photographic Light Sensitive Material," U.S. Pat. No. 4,201,586 (1980).
R-16. P. Bagchi and W. L. Gardner, "Use of Gelatin-Grafted and Case-Hardened Gelatin Grafted Polymer Particles For Relief From Pressure Sensitivity of Photographic Products," U.S. Pat. No. 5,026,632 (1991).
Gelatin has been used as the primary peptizer for the precipitation of silver halide grains and also as a coating vehicle in conventional photographic recording materials for over 120 years and remains one of the most important components of photographic systems (R-1, R-2). Cattle (cow) bones are the principal starting material for photographic gelatin. Sometimes, cattle and pig skins are also used. However, skin gelatins usually contain photographically active components and, therefore, their uses in photographic systems are limited (R-2). The manufacture of gelatin involves several stages. The first step is the deashing process to reduce the calcium (mainly calcium triphosphate or calcium apatite and calcium carbonate) content of the bones through a soak for about a week in a mineral acid bath. This decalcified material is referred to as collagen or "ossein."
Collagen or the ossein is a crosslinked and structured polypeptide (R-1), ##STR1## which is further treated either by lime or by a mineral acid to hydrolyze and denature the tertiary, secondary and partly the primary structures to produce water-soluble gelatin according to the schematics of FIG. 1. During the formation of gelatin collagen, which is composed of crosslinked triple helices of .alpha..sub.1 and .alpha..sub.2 chains (MW=285,000), is first denatured to the randomly coiled .gamma. form, then to a mixture of the .beta..sub.11 (composed of two .alpha..sub.1 chains MW=190,000), .beta..sub.12 (composed of one .alpha..sub.1 and one .alpha..sub.2 chain, MW=190,000), and to single .alpha..sub.1 and .alpha..sub.2 stands (MW=95,000) and sub-alpha fragments (MW&lt;95,000). The solubilized gelatin fractions are leached and, for many applications, deionized by passage through ion exchange beds, chilled, noodled, and then dried for storage. Lime processing to produce gelatin requires between 2 to 3 months of treatment, whereas acid treatment usually needs several days (R-2). Consequently, for a manufacturing procedure, acid processing is definitely less expensive compared to lime processing and thus economically attractive. However, since acid hydrolysis occurs more rapidly, it is less controllable, and it leads to gelatins that usually have much lower average molecular weights than those derived from lime treatment. As a result, those gelatins may not provide adequate steric stabilization (R-3, R-5) to the emulsion grains. For this reason, an acid processed ossein (APO) gelatin was intermolecularly crosslinked or chain-extended (CE) (R-4) to produce a gelatin sample with viscosity (hence, effective molecular weight) comparable to standard lime-processed gelatin.
Dispersions of silver halide microcrystals (often referred to as emulsions in the photographic literature) with narrow grain-size distribution are usually precipitated by the so-called "double-jet" precipitation technique. Those emulsions usually contain gelatin as peptizer and steric stabilizer, and they make use of solutions of .about.4 M AgNO.sub.3 and 4 M halide salt solutions. Therefore, under precipitation conditions, all electrical double layer effects on the stability of the silver halide emulsions are virtually negligible. However, the halide ion or the silver ion concentration during the precipitation process has a profound effect on the morphology of the crystals formed during the precipitation process. In a double-jet precipitation device (FIG. 2, to be described later and in R-6), the concentration of silver ion (or halide ion, the two being related by the solubility product of silver halide) can be measured by a silver electrode and can be maintained at specific pAg (=-log.sub.10 [Ag.sup.+ ]) values. The particle nucleation and growth process that take place at different silver ion or halide ion excesses, produce microcrystals of different morphology. In the case of AgBr, at pBr values greater than 3 generally cubic crystals are obtained, between 2 and 3 usually octahedral crystals are formed, and at pBr values below 2 platelets or tabular emulsion crystals are usually generated (R-1). However, from the colloid stability point of view, the particle size and crystal morphology are very important as they both determine the extent and the functional form of the van der Waals' attraction and steric or electric double layer repulsion (R-3, R-5).
The stability of a sterically stabilized colloidal system is primarily determined by the conformation of the adsorbed macromolecule on the particle surface and the resulting hydrodynamic thickness of the adsorption layer (R-1, R-8). For amphoteric polyelectrolytes, like gelatin and other proteins that exhibit a pH corresponding to zero net charge (PZC), the extent of the adsorbed amount (F) is usually highly pH dependent (R-8). That phenomenon has been attributed to the ionization induced expansion or contraction of isoelectric proteins. Measurements of the adsorbed layer thickness (L) by inelastic light scattering allows the characterization of the colloidal stability criterion of sterically stabilized dispersed systems.
As indicated above, the physical properties of gelatin, such as PZC, molecular weight and molecular weight distribution depend upon the nature of the processing, such as lime or acid. It has generally been noted that the PZC of lime processed (ossein) gelatin is around pH of 4.9 (R-7) and that of acid processed pigskin gelatin is much higher around pH of 9.1 (R-7). Using a series of acid processed ossein gelatins (APO) that have been processed in acid over 1,3,4 and 10 days, Toda (R-2) demonstrated that the APO gelatins have a much higher PZC and a broader distribution of PZC compared to lime processed gelatins. However, longer acid treatment times lead not only to sharpening of the PZC distribution, but also to movement of PZC to much lower pH. As indicated above, longer acid treatment produces gelatins with much lower molecular weight distributions, and those gelatins are generally not considered suitable for adequate peptization of silver halide crystals used in photography. Higher molecular weight (short acid treatment) acid processed gelatins (like pigskin gelatin) have been used infrequently in preparation of photographic emulsions because of the high PZC (R-7). High PZC Ag-halide peptizers under many precipitation conditions (pH and pAg) can lead to sensitized flocculation rather than peptization depending upon the PZC (pAg) of the silver halide salt in question. A further disadvantage of high PZC gelatins is that they coacervate when mixed with regular lime processed ossein gelatin under normal coating pH conditions 3-8. Therefore, the cost advantage of acid processed ossein gelatins can be exploited only if acid processed gelatins have sufficiently high molecular weight and reasonable low PZC to avoid adverse colloid chemical interactions. Therefore, there is a need for an invention that will render relatively inexpensive acid processed ossein gelatin usable in photographic systems.
Even though acid processed gelatins have not found extensive use in photographic systems because of the problems outlined above, limited disclosure has been located of its use in overcoats, away from the silver halide containing sensitized layers and in the interlayers between the sensitized layer [Nagamoto et al., U.S. Pat. No. 4,266,010 (R-13); 4,021,214 (R-14); and Hozi et al., U.S. Pat. No. 4,201,586 (R-15)]. It has been reported in those publications that when acid processed gelatins, extended acid processed gelatins or acid processed gelatins in combination with standard lime processed gelatins, are utilized to produce photographic overcoat layers, the photographic products have greater abrasion resistance. In those disclosures, the term "extended gelatin" has been defined as
"Gelatin that has been chemically modified by grafting onto it small molecules or other polymers, via the gelatin amine, immine or carboxyl groups, to form either a water soluble or water dispersible polymeric or colloidal product."
Also such gelatins in the prior disclosures are in general acid processed skin gelatins as opposed to acid processed ossein gelatins.